1. Technical Field
The present invention relates to a fixing device for use in an image forming apparatus such as a printer, a facsimile machine, a copier, and the like, and to an image forming apparatus incorporating such a fixing device.
2. Related Art
Conventionally, an image forming apparatus is configured such that a latent image formed on an image carrier based on image data is developed by toner supplied from a developing device so that a visible toner image is formed on the image carrier. The toner image on the image carrier is transferred onto a recording medium by a transfer device, and is fixed on the recording medium by the fixing device.
JP-H06-138793-A discloses a fixing device in which a rotatably-mounted pressure roller and a flexible endless fixing belt are pressed against each other to form an area of contact herein referred to as a nip or nip portion. The thus-formed fixing device includes a heating member connected to a power source and disposed at an interior side of the fixing belt that forms the nip portion. The heating member contacts the interior surface of the fixing belt and heats the fixing belt electrically, and includes a plurality of heat sources arranged along the width of the sheet perpendicular to a sheet conveyance direction. The heating sources are selectively controlled so that the area heated by each heat source is changed based on the image data and corresponding to an unfixed image on the sheet. As a result, the fixing belt is heated by the heating member so that only that portion of the fixing belt bearing the unfixed toner image is heated. The unfixed image on the sheet that has been conveyed to the nip portion is then fixed onto the sheet with heat and pressure. Compared to a case in which an entire fixing belt is heated up to the fixing temperature, such an arrangement saves energy.
However, heat transfer from a rear side to a surface side of the fixing belt heated by the heating member requires time. Thus, there is a delay between the time the heating member is activated and the time at which the fixing belt attains the desired fixing temperature, thereby possibly causing a fixing error due to insufficient heat. Accordingly, based on the image data of the unfixed image existing in the nip portion after one rotation of the fixing belt, the fixing belt is again heated by the heating member by previously changing the area to be heated by each heat source one rotation of the fixing belt before the unfixed image exists in the nip portion. Accordingly, a predetermined amount of heat can be transferred to the surface of the fixing belt by the time the fixing belt rotates one cycle and the heated portion of the fixing belt again arrives at the nip portion, thereby preventing a fixing error from occurring due to insufficient heat.
However, because the temperature of the fixing belt decreases due to dissipation of the heat from the surface of the fixing belt during rotation of the belt, a large amount of heat needs to be supplied from the heating member to the fixing belt which in turn necessitates heavy energy consumption.